The present invention relates generally to an apparatus and a method for vaporizing a liquid. More in particular, it relates to an apparatus and a method for vaporizing a mixture of liquefied gases in chemical equilibrium with a binary compound thereof.
Though the invention is applicable to the vaporization of various mixtures of liquefied gases, it will be explained in detail in connection with bromine chloride (BrCl), for this compound is becoming increasingly important as a brominating agent, oxidizing agent, and disinfectant.
Although chemists have long been familiar with many of the properties of bromine chloride, industrial processors and other users have displayed a reluctance to employ bromine chloride despite its obvious and pronounced advantages over either chlorine or bromine in many applications. This reluctance stems partly from the lack of handling and metering technology capable of precisely and predictably withdrawing uniform compositions of liquid bromine chloride from storage vessels, and introducing vaporized bromine chloride of substantially the same composition to a gas dispensing system. There is, therefore, a need for a vaporizing apparatus capable of efficiently, effectively, and accurately supplying gaseous bromine chloride for various industrial end uses.
A number of devices for converting a single component liquefied gas, such as chlorine or hydrogen, to a superheated gas are commercially available. Two such devices are described, for example, in U.S. Pat. Nos. 3,949,565 and 3,346,718.
In the operation of such devices, a single component liquefied gas is heated to boiling and converted to a superheated gas which is then discharged from the device. The heating is commonly accomplished by use of a heating element which is indirectly heated and is in continuous direct physical contact with the liquefied gas. For example, in U.S. Pat. No. 3,949,565 a heating element extends upward through the base of the device and contacts the reservoir of liquefied gas contained in the interior thereof.
A number of drawbacks have been encountered in attempting to employ these known devices generally, and more particularly in the vaporization of liquefied gas mixtures such as bromine chloride. For example, configurations wherein the heating element and the reservoir of liquefied gas are continuously in direct contact have proven undesirably corrosive to the heating element. Additionally, the liquefied gases that are vaporized usually contain a significant amount of nonvolatile residue. During vaporization, these residues tend to deposit at the bottom of the liquefied gas reservoir. When the heating element is in continuous direct contact with the reservoir, the nonvolatile residues deposit on the heating element causing a substantial decrease in the efficiency of heat transfer from the heating element to the contents of the reservoir. The build-up of nonvolatile residues also accelerates the rates of corrosion of the metal materials from which heating elements are commonly constructed.
Another drawback commonly encountered in using the commercially available devices is the difficulty in maintaining an unimpeded flow of liquid into the device. The most common problem occurs where the liquid entering the device is subjected to a pressure drop. As a result of the pressure drop, the liquefied gas feeding into the device prematurely vaporizes and deposits the nonvolatile residue in the feed lines to the reservoir and eventually plugs the system.
Another common problem in most vaporizer systems is the reliquefaction of the vaporized gas leaving the vaporizer. Reliquefaction of the gas causes plugging problems on the gas side of the vaporizer and also can create metering inaccuracy and potential safety hazards.
However, the major disadvantage of commercially available vaporizing devices is that they are not adapted to vaporize a mixture of two liquefied gases in chemical equilibrium with a binary compound thereof, such as bromine chloride. In the conventional devices where only a single component liquefied gas is being vaporized, the vapor above the liquefied gas reservoir in the apparatus will generally have the same chemical composition as the supply source regardless of changes in reservoir levels caused by temperature variation. Hence, the vapor removed from the vaporizer will have the same chemical composition as the liquid introduced into the vaporizer.
The situation changes when the liquid to be vaporized is a mixture of mole fractions of two liquefied gas components in chemical equilibrium with a binary compound of those gases. Then the vapor above the liquid gas reservoir in the vaporizer can have a composition substantially different from the supply source due to the complex liquid-vapor equilibrium established when two components of relatively different volatilities are present in both the liquid and vapor phase. In most applications, it is undesirable to withdraw a vaporized gas having a composition different from that of liquid feed material to the vaporizer.
The particular difficulties involved in vaporizing a mixture of two liquefied gases in chemical equilibrium with a binary compound thereof are best illustrated by bromine chloride. It is well-known that bromine chloride molecules exist in chemical equilibrium with the parent bromine molecule and chlorine molecule in both the gas and liquid phases in an equilibrium of the type 2 BrCl.revreaction.Br.sub.2 +Cl.sub.2. Consequently, the liquid bromine chloride from which the gas must be vaporized is an equilibrium solution of equimolar amounts of molecular bromine and molecular chlorine each exerting its own characteristic vapor pressure.
In considering the equilibrium set up within a vaporizer at a particular pressure and temperature, between a two-component bromine chloride liquid solution and its vapor, it is helpful to consult a boiling-point composition diagram. In such a diagram, the pressure is fixed and the temperature at which the liquid and vapor are in equilibrium is plotted as a function of the mole fraction of the least volatile component. A typical boiling point diagram for bromine chloride is described by G. H. Cheesman, and D. L. Scott in Australian J. Chem, 1968, 21, p. 289-97. The composition of the vapor and the composition of the liquid in equilibrium with it are shown on the same diagram. The vapor composition curve of the least volatile (bromine) component lies above the liquid composition curve. Consequently, at a given temperature (construct a horizontal line across the diagram), the vapor and liquid have different compositions, the liquid always being richer in the least volatile component (bromine). Consequently, the simple evaporation of a liquid bromine chloride pool would result in the initial removal of a vapor rich in the more volatile chlorine component. At the same time, the liquid composition would continually be changing until only bromine would be left to evaporate. It would be difficult to provide a bromine chloride gas of constant composition by the simple evaporation of a pool of bromine chloride.
It is therefore desirable to develop a vaporizer for vaporizing a mixture of two liquefied gases in chemical equilibrium with a binary compound of those gases. The vaporizer must be capable of vaporizing a gas having substantially the same composition as the liquid feed. In addition it should avoid plugging of the feed lines, excessive corrosion of the the element, and reliquefaction of vapor all of which problems are present in conventional vaporizing devices. The apparatus of the present invention achieves these desirable results.
For the purposes of this specification, a vapor is defined as a gaseous substance having liquid suspended therein. While a gas is defined as a gaseous substance having substantially no liquid suspended therein.